Structural and transport properties of V_6O_1_3 insertion electrodes

Polycrystalline V_6O_1_3 with an apparent oxygen excess stoichiometry was synthesised by controlled thermal decomposition of ammonium metavanadate and chemically lithiated with n-butyl lithium. X-ray diffraction was used to characterise the prepared powders and lattice parameters were determined as a function of lithium content. Examination of the unlithiated host material by high resolution electron microscopy revealed a complicated array of shear and twin defects. Single crystals of V_6O_1_3 were grown by chemical vapour transport and lithiated by coulometric titration in solid state electrochemical cells. From the shape of the coulometric titration curves a single phase region was identified for Li_xV_6O_1_3 between x = 0 and x 0.17 at 120 deg. C. Chemical diffusion coefficients were estimated over this composition range in two perpendicular crystallographic directions. The figures obtained were independent of composition with mean values of 3.5 x 10"-"8 and 4.9 x 10"-"9 cm"2 s"-"1 at 120 deg. C perpendicular to the (010) and the (001) crystal planes respectively. The variation in cell voltage between 80 and 130 deg. C was investigated and found to show two linear regions with a change in slope occurring between 110 and 100 deg. C. Values of the partial molar free energy of lithium were calculated at 120 deg. C. The thermodynamics of the Li-V_6O_1_3 system between x = 0 and x = 7.7 were examined by measuring the emf of cells, constructed with chemically lithiated polycrystalline cathodes, between 65 and 135 deg. C. High partial molar enthalpies (from -270 to -215 kJ mol"-"1) and low partial molar entropies (around -25 kJ mol"-"1 K"-"1) were determined. All cells were constructed using lithium trifluoromethane sulphonate poly(ethylene oxide) electrolyte which was first characterised by ac impedance spectroscopy. Lithium ion transport numbers at the compositions O/Li = 4, 4.5 and 8 were found to be only about 0.45. (author)Available from British Library Document Supply Centre-DSC:DX210887 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo